selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE

This validates that GS selector and base are independently preserved in
ptrace commands.
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200528201402.1708239-17-sashal@kernel.org

tools/testing/selftests/x86/fsgsbase.c

@@ -465,7 +465,7 @@ static void test_ptrace_write_gsbase(void)
 	wait(&status);
 
 	if (WSTOPSIG(status) == SIGTRAP) {
-		unsigned long gs;
+		unsigned long gs, base;
 		unsigned long gs_offset = USER_REGS_OFFSET(gs);
 		unsigned long base_offset = USER_REGS_OFFSET(gs_base);
 
@@ -481,6 +481,7 @@ static void test_ptrace_write_gsbase(void)
 			err(1, "PTRACE_POKEUSER");
 
 		gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
+		base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
 
 		/*
 		 * In a non-FSGSBASE system, the nonzero selector will load
@@ -501,8 +502,14 @@ static void test_ptrace_write_gsbase(void)
 			 */
 			if (gs == 0)
 				printf("\tNote: this is expected behavior on older kernels.\n");
+		} else if (have_fsgsbase && (base != 0xFF)) {
+			nerrs++;
+			printf("[FAIL]\tGSBASE changed to %lx\n", base);
 		} else {
-			printf("[OK]\tGS remained 0x%hx\n", *shared_scratch);
+			printf("[OK]\tGS remained 0x%hx", *shared_scratch);
+			if (have_fsgsbase)
+				printf(" and GSBASE changed to 0xFF");
+			printf("\n");
 		}
 	}